Machines of all sorts rely upon refined petroleum products, such as gas and motor oil, in order to operate. The increasing number of machines built and sold each year ensures that the amount of fuel supplied in a given period of time will eventually not be able to support all the vehicles in operation. Additionally, there are significant and wide-spread concerns about the environmental aspects of fossil fuels attributed significantly to global warming. Fossil fuels are a non-renewable resource having only a finite supply which has sparked concern about energy shortages or a world-wide energy crisis if fossil fuel production ceased or otherwise lagged behind demand. Therefore, alternative energy and fuel research is an important and competitive industry.
Natural gas is one of the cleanest burning fossil fuels, and millions of vehicles worldwide have been modified or built to run on it. In fact, the infrastructure to support the use of natural gas has been developed in some areas where its purer combustion properties are highly valued. Unfortunately, there are a number of drawbacks to using natural gas as a transportation fuel. First, natural gas is still a non-renewable resource. The finite supply of natural gas means the price fluctuates with production. In general, natural gas is not an economically competitive alternative for most consumers. Also, burning natural gas still contributes to global warming gases. Finally, the energy density at which combustion occurs is over one thousand times less than conventional liquid fuels. In order to overcome its low energy density, natural gas must be highly pressurized. High pressures must be combined with low temperatures in order to convert natural gas into a dense, easily transported liquid fuel.
Natural gas mainly consists of methane (CH.sub.4), but, depending on the terrestrial origin of the gas, it can contain other trace gases such as hydrogen sulfide, hydrogen, propane, butane, etc. While natural gas is a non-renewable resource, methane is generated as a natural by-product of anaerobic fermentation or digestion, which is a ubiquitous environmental process essential for reducing organic matter in the natural environment. The main by-products of anaerobic digestion are methane, at generally one-half to two-thirds of the resulting gas, and carbon dioxide, along with trace levels of hydrogen sulfide, oxygen, nitrogen and water vapor. Almost all of the energy in the original biodegradable organic matter is contained in this renewable source of methane.
One alternative to the heavy reliance on fossil fuels involves purifying the gas that results from anaerobic digestion, also known as “biogas,” in order to produce a pure, renewable methane stream. Typically, anaerobic digestion devices (i.e., anaerobic digestion that is not occurring in nature) are intended to convert organic material, also known as “biomass,” from one form to another. For example, biomass can be placed in a silo for partial fermentation that converts the biomass to animal feed. Anaerobic digestion is also used to treat plant, animal and human waste. These waste materials can be converted into a fertilizing material. Yet, methane produced from anaerobic digestion would still need to be compressed to greater than 2000 pounds/inch.sup.2 (2000 ‘psi’) in order to approach the energy density of conventional liquid fuels. Even at 2000 psi, methane is a gas, and it would need to be purified, for some applications, before being used as a fuel. Known biogas purification and compression methods and apparatuses can not produce a cost-effective fuel. As such, methods and devices for producing biogas from anaerobic digestion have been rejected as viable alternatives for the production of fuel. A suitable process would provide a renewable fuel source while treating waste products that must otherwise be disposed of as well as being capable or using most sources of photosynthetically fixed biomass.
The fuel in biogas powered machines uses the same engine configuration as natural gas machines. The gas quality demands are strict. The raw biogas from a digester need to be upgraded in order to obtain biogas which: 1) has a higher calorific value in order to provide more energy output; 2) has a regular/constant gas quality to obtain safe operation of the machine utilizing the biogas as an energy source; 3) does not enhance corrosion due to high levels of hydrogen sulfide, ammonia, and water; 4) does not contain mechanically damaging particles, 5) does not give ice-clogging due to a high water content and 6) has a declared and assured quality. In practice, this means that carbon dioxide, hydrogen sulfide, ammonia, particles and water (and other trace components) have to be removed so that the product gas for vehicle fuel use has methane content above 95%. Different quality specifications for vehicle fuel use of biogas and natural gas are applied in different countries.
A number of biogas upgrading technologies have been developed for the treatment of different sources of biogas, such as natural gas, sewage gas, landfill gas, etc. At present, four different methods are used commercially for removal of carbon dioxide from biogas either to reach vehicle fuel standard or to reach natural gas quality for injection to the natural gas grid.
Primarily, water scrubbing is used to remove carbon dioxide but also hydrogen sulfide from biogas, since these gases are more soluble in water than methane. The absorption process is purely physical. Usually the biogas is pressurized and fed to the bottom of a packed column where water is fed to the top so the absorption process is operated counter-currently. The water which exits the column with absorbed carbon dioxide and/or hydrogen sulfide can be regenerated and re-circulated back to the absorption column. The regeneration is made by depressurizing or stripping with air in a similar column. Stripping with air is not recommended when high levels of hydrogen sulfide are handled since the water will soon be contaminated with elementary sulfur which causes operational problems. The most cost efficient method is not to re-circulate the water if cheap water can be used, for example, outlet water from a sewage treatment plant.
However, this purification process has a number of limitations, in particular, the inability to significantly remove the O.sub.2 and/or the N.sub.2 components of the biogas. The O.sub.2 and N.sub.2 present in the biogas can build up over time and negatively affect the purity of the natural gas, making unsuitable for introduction directly into a natural gas pipeline. In addition, the amount of water that is necessary to enable the stripping process to operate effectively is very high, which makes the utilization of suitable water-regeneration processes undesirable.
Therefore, there exists a need for a system for methane production using biogas produced as the result of anaerobic digestion or other similar processes that can sufficiently remove not only the primary contaminants from the biogas, such as carbon dioxide and hydrogen sulfide, but also more trace impurities, e.g., O.sub.2, N.sub.2, and moisture to produce a pipeline quality natural gas.